1. The Field of the Invention
The present invention relates generally to the dissipation of heat from packaged die. More specifically the present invention relates to a novel package having a highly thermally conductive housing that improves the removal of heat generated by a die during use.
2. The Relevant Technology
Energized circuits consume electric power and convert that power into other forms of energy. Such other forms include sound energy, electromagnetic radiation and chemical energy. The most common conversion is that of electric power into heat through some form of resistance. Sometimes this heat is generated on an almost continuous basis such as with circuits using bipolar devices. Other times this heat is generated intermittently such as with CMOS devices which only generate heat in proportion to their operating frequencies.
Whether continuous or intermittent, electric power lost to heat in electrical devices sometimes serves a useful purpose, such as in a toaster or a hair dryer. Most times, however, heat generation is unintended and causes significant problems. Some problems are subtle, like modifications in the operating characteristics of the components such as reductions in speed and variations in temperature coefficients. Yet, some problems are more obvious, like mechanical failure of components due to explosion, melting or mismatches that occur in their coefficients of thermal expansion. In either instance, these heat related problems adversely impact upon the failure rates of components and severely shorten the lifetime of any circuits using them.
Instead of improving, heat related problems are worsening. In part, this is because of the ongoing quest for higher concentrations of circuits in the same or smaller sized areas. This higher concentration or density of circuits per unit area is best exemplified in ever-evolving devices such as supercomputers, hand-held devices, consumer electronics and minicomputers. With this evolution, however, not only is the physical size of the circuit decreasing but so too is the physical size of the internal components. This leads to the generation of even more heat.
In general, heat is removed from circuits and components by either active or passive means. Active means include, but are not limited to, forced air fans and circulating liquids like water and boiling fluids. Although the active means are effective at removing heat, they have limitations. One limitation is the expense in design, manufacturing and implementation. Another limitation is complexity. For example, circuits using forced air fans must have adequate space for the fan housing, which is typically much larger than the components of the circuit. These circuits must also have the fan positioned to best remove the heat which requires extensive pre-manufacturing analysis.
Passive means of heat removal include, but are not limited to, layout designs to allow for improved ventilation and heat sinks. Heat sinks, however, are problematic because of their typically enormous fin size in comparison to the components generating the heat. In turn, this causes circuits to maintain a large physical space which is adverse to the continual trend of miniaturization and downsizing. Moreover, heat sinks often vary in their shape from component to component. This variation causes problems when replacing worn components because circuits are not designed to adjust to each of the various heat sink shapes.
Still other heat removal means are taught in the prior art. In U.S. Pat. No. 5,702,985 issued to Burns, heat removal is taught by reducing the thickness of a die and the materials between the die and a heat conductive lead frame. Although this method tends to create a more streamlined package, this method relies exclusively on reducing component thicknesses within the package. This disadvantageously requires unique sized internal components to be manufactured and cannot easily find compatibility with other internal components.
Burns also teaches a ceramic housing for these components. Although ceramics are better thermal conductors than plastics, ceramics generally do not equal the thermal conductivity characteristics of pure metals and metal alloys. Therefore, Burns necessarily relies on a component other than the housing to ultimately conduct heat away from the die during use. In particular, the metal lead frame is used.
Although some prior art packages use metal as a housing, the die within the housing is typically thermally insulated from the metal by molding compounds. As such, the metal is used primarily as a barrier against mechanical and environmental problems such as vibration, electromagnetic radiation and moisture.
Accordingly, it is desirous to improve the removal of heat from a packaged die by employing the best known thermal conductors while still allowing for improvements in miniaturization, replacement conformity and costs.
In accordance with the invention as embodied and broadly described herein, a novel die package is provided by a semiconductive substrate having a plurality of electrical circuits fabricated on a circuit side thereof and having an opposite backside thereof.
In the context of this document, the term xe2x80x9csemiconductor substratexe2x80x9d is defined to mean any construction comprising semiconductive material, including but not limited to bulk semiconductive material such as a semiconductive wafer, either alone or in assemblies comprising other materials thereon, and semiconductive material layers, either alone or in assemblies comprising other materials. The term xe2x80x9csubstratexe2x80x9d refers to any supporting structure including but not limited to the semiconductor substrates described above. The term semiconductor substrate is contemplated to include such structures as silicon-on-insulator, silicon-on-sapphire, gallium arsenide (GaAs), and Germanium.
A heat block also provides and is composed of a material having a thermal conductivity of not less than about 1.3 W/cmxc2x0 C. The heat block has an external surface and a cavity therein. The cavity has a mounting surface therein to which the backside of the semiconductor substrate is adhered and through which heat from the backside of the semiconductor substrate is conducted to the external surface of the heat block. An electrically insulative encapsulant encapsulates the semiconductive substrate within the cavity of the heat block and an electrical connector is in electrical communication with the plurality of electrical circuits and extending through the encapsulant. The encapsulant can also be chosen to be a good thermal conductor.
The inventive die package preferably has a highly thermally conductive housing. The housing is a heat block, preferably formed substantially of metal, which is abutted against the die in a thermally conductive manner to facilitate the transfer of heat away from the die during use.
As described herein, the heat block is a substantially rectangular mass of metal with a cavity formed therein for receiving the die. In one embodiment, the cavity is substantially rectangular and bound on three sides by a wall and open on the fourth side. In another embodiment, the cavity is substantially rectangular and bound on all four sides by a wall. In both embodiments, the cavity is slightly larger than the perimeter of the die and the walls are arranged for close proximal contact with the perimeter.
A metal lid is provided for at least partially covering the die and for thermally conductively attaching to the die to even further facilitate heat transfer. In one embodiment, the lid has holes arranged therein to allow electrical access to the die. The holes afford footprints of the die to be shaped in a ball grid style arrangement. In another embodiment, the lid is solid and allows a lead frame to connect the die to a circuit by traversing underneath the lid.
A metal socket is provided to align the package to a circuit. The socket is designed for thermal conductive contact with the heat block when the heat block is fully seated. The socket is configured with channels for matingly receiving the lead frame of the package. In one embodiment, the socket has channels that allow the direct throughput of the lead frame to the circuit. In another embodiment, the channels are electrically coupled to the circuit by a socket pin. The socket may alternatively be configured with a plurality of heat sinks that simultaneously thermally conductively contact the socket and the heat block of the package.
The foregoing embodiments result in a novel heat dissipating package that still allows for miniaturization, saves on materials costs, allows a conforming fit with existing circuits without causing costly redesign, and utilizes the best known thermal conductors to dissipate heat instead of relying on ceramics, polymers or plastics.
These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.